The technology of the ultra wideband type differs from narrowband and spread spectrum technologies. An exemplary definition of technology of the ultra wideband type is provided by the Federal Communications Commission (FCC). This organization specifies that the bandwidth of the signal at −10 dB is greater than 500 MHz and/or greater than 20% of the center frequency.
Moreover, instead of transmitting a continuous carrier modulated with information or with information combined with a spreading code, thereby determining the bandwidth of the signal, the ultra wideband technology provides for the transmission of a series of very narrow pulses. For example, these pulses may take the form of a single cycle or monocycle, having a pulse width of less than 1 ns, for example. These extremely short pulses in the time domain, when transformed into the frequency domain, provide the ultra wideband spectrum characteristics of UWB technology.
In UWB technology, the information conveyed on the signal can be coded by pulse position modulation (PPM), for example. Stated otherwise, the coding of information is performed by altering the instant of transmission of the individual pulses. More precisely, the pulse train is transmitted at a repeat frequency that may be as much as several tens of MHz. Each pulse is transmitted in a window of predetermined length, for example, 50 ns. With respect to a theoretical position of transmission, the pulse then leads or lags. This makes it possible to code a 0 or a 1. It is also possible to code more than two values by using more than two positions shifted with respect to the reference position. It is also possible to superimpose a BPSK modulation on this positional modulation. It is even possible to perform a frequency modulation.
Given the center frequency of the pulses, which is generally on the order of a few GHz, and a positional shift of the pulses with respect to the theoretical position which is, for example, on the order of a few tens of picoseconds in a PPM modulation, it then becomes necessary to use clock signals having very high frequencies, for example, on the order of about 100 GHz. This requires an approach that is constraining both from a technical point of view and from a current consumption point of view. Thus, it is very difficult to embody this approach in CMOS technology.
Moreover, it is important that the accuracy of the clock is also very good, typically a few picoseconds, thereby further adding to the technical constraints. Furthermore, embodying conventional pulse generators controlled by signals having very high frequencies proves to be particularly tricky, especially at the level of the control of the actual position of the pulses, of their center frequency and of their duration.